1. Field of the Invention
The present invention relates to an optical scanner and an image forming apparatus that includes the optical scanner, and more particularly, to an optical scanner that optically scans a surface by using an optical deflecting unit.
2. Description of the Related Art
In a tandem color image forming apparatus, each optical beam of a plurality of light beams emerged from each of a plurality of light sources is irradiated on each of a plurality of image carriers to form a latent image on each image carrier. Each latent image is developed with a toner of different color, such as black, magenta, cyan, and yellow, to obtain a visible image of corresponding color. The visible images formed on the image carriers are transferred to a transfer material one above the other. As a result a multicolor image is formed on the transfer material.
Some tandem color image forming apparatuses employ a method of performing exposure scanning by each optical scanner provided for each image carrier. However, in this case, a polygon mirror is required in each optical scanner, and also a rotating deflector (optical deflector) is required. Moreover, a motor is required for driving the polygon mirror. As a result, such tandem color image forming apparatuses become costlier and bulky.
One approach is to use only one rotating deflector. Tandem color image forming apparatuses that use only one rotating deflector have been disclosed in Japanese Patent Application Laid-open Publication Nos. H10-148781, 2002-196269, and 2003-202515.
Problems involved in the conventional method are described with an example of an optical writing unit (hereinafter, “optical scanner”) described in Japanese Patent Application Laid-open. Publication No. 2002-196269, with reference to FIG. 8 and FIG. 9. In both FIGS. 8 and 9, reference numeral 500 denotes a conventional optical scanner. In the optical scanner 500, with a single rotating deflector 62 at a center, light source units 52, 53, 54, and 55 which include a light source at substantially symmetrical positions with respect to the rotating deflector 62, and first lenses for image forming (hereinafter, “scanning lenses”) 63 and 64 are disposed. The exposure scanning is performed by distributing light beams L1, L2, L3, and L4 from the light source units 52, 53, 54, and 55 respectively to left and right sides.
The two light beams each from among the light beams L1 to L4 from the four light source units 52 to 55 are distributed in two symmetrical directions by the rotating deflector 62 that rotates in a direction of an arrow in FIG. 8, and are subjected to deflection scanning. The light beams subjected to the deflection scanning are formed as images on surfaces to be scanned (not shown in the diagram) of the four image carriers via a scanning optical system that includes the scanning lenses 63 and 64, and mirrors 65, 66, 67, and 68 for reflecting an optical path. In FIG. 8, reference numeral 95 denotes a wall surface of light shielding members 89 and 90.
In the optical scanner 500, the rotating deflector 62 can be let to be a single rotating deflector, thereby enabling to reduce the cost and the size of the optical, scanner as compared to a size of an image forming apparatus that includes a plurality of optical scanners. However, in the optical scanner (so called opposite-side scanning scanner) having such structure, as shown in the diagram, with the rotating deflector 62 as a center, since each optical system is disposed symmetrically, flare light from the optical systems that are mutually opposite sometimes causes a problem. For example, when a light beam is reflected and scattered at a side of a surface of incidence of the scanning lens 63 and 64 that are mutually opposite sandwiching the rotating deflector 62, stray light becomes the flare light and advances in a reverse direction entering into an optical system on the opposite side. This stray light is irradiated on an image carrier that is not supposed to be subjected to the exposure scanning, and becomes a ghost image or an image with lines, thereby substantially deteriorating an image quality.
Japanese Patent Application Laid-open Publication Nos. H10-148781 and 2003-202515 propose a technology to solve the above problems.
For example, a technology disclosed in Japanese Patent Application Laid-open Publication No. 2003-202515 is related to an optical scanner that is capable of cutting the flare light effectively without deteriorating jitter (fluctuation) in a main scanning direction and noise of the polygon mirror and forming a high quality image by using simple components such as plastic lenses, and a color image forming apparatus that uses this optical scanner. In this technology, the flare light is prevented by providing a light shielding member in an effective area sandwiched between an optical path of a light beam that is incident on the rotating deflector and an optical path of a scanning beam that is reflected and deflected at the rotating deflector and incident on a surface to be scanned. By providing the light shielding member at such location, it is possible to dispose the light shielding member separated away from the rotating deflector and to reduce wind noise caused due to rotations of the rotating deflector.
A technology disclosed in Japanese Patent Application Laid-open Publication No. 2002-196269 is related to an optical writing unit and an image forming apparatus that is capable of preventing an entry of the flare light into an optical system on the opposite side, even when the flare light is generated by optical systems opposite to each other sandwiching the rotating deflector, in an optical writing unit that performs deflection scanning of a plurality of light beams by one rotating deflector by distributing the light beams in two symmetrical directions. This technology is characterized by providing the light shielding members 89 and 90 as shown in FIG. 8 in an area other than an area that is subjected to deflection scanning, by a single rotating deflector.
Moreover, a technology disclosed in Japanese Patent Application Laid-open Publication No. H10-148781, is related to a light beam scanner that is capable of preventing and reducing stray light that is reflected from one of two scanning and imaging optical systems that are disposed symmetrically with respect to a rotating polygon mirror with an optical axis of the optical systems substantially parallel to be incident on the other scanning and imaging optical system and becoming noise.
However, in the technology disclosed in the Japanese Patent Application Laid-open Publication No. 2003-202515, for preventing the entire flare light, it is necessary to dispose the light shielding members very near to the rotating deflector and for shielding only the flare light without shielding the essential light beam, a very high component accuracy is required, which results in a high cost of components.
In addition, with an arrangement of the light shielding member according to an embodiment of this technology, for shielding the entire flare light, since the shielding member has to be still disposed near the rotating deflector, the problem of the wind noise due to the rotation of the rotating deflector remains to be there.
Moreover, in this case, not only the wind noise but also a rise in temperature near the rotating deflector caused due to a windage loss that occurs due to the rotation of the rotating deflector is a problem. In other words, by providing the light shielding member near the rotating deflector, a pressure of an air flow is increased locally and as a result of this, there is a rise in the temperature caused due to the windage loss. Such rise in temperature near a portion where the rotating deflector is installed, deteriorates a stability of operation and life span of the rotating deflector, as well as deteriorates color superimposing accuracy (so called color shift) and an imaging efficiency of a scanning optical system due to a transmission and a propagation of heat to the scanning optical system, thereby deteriorating remarkably the image quality. In particular, this effect is extremely greater in a color image forming apparatus that includes this optical scanner.
Moreover, in the technology disclosed in the Japanese Patent Application Laid-open Publication No. 2002-196269, regarding the noise which is one of the problems mentioned, the technology is characterized by employing a structure in which a gap that allows the air flow to escape is provided in the light shielding member. However, since the light shielding member is disposed near the rotating deflector, the problems of noise and rise in temperature still have not been solved completely.
In addition, in a case of forming the light shielding member in this technology integrally in an optical housing that includes the rotating deflector, if a case in which a cheap resin molded product is used as the optical housing is assumed, a die structure becomes complicated (a portion corresponding to a light shielding wall of the light shielding member is under cut or forced cut) and may result in a rise in the cost.